Superposition in Quantum Computers - Computerphile
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- เผยแพร่เมื่อ 25 ต.ค. 2021
- Parting the veil of mystery on quantum superposition using waves. Professor Phil Moriarty takes us through it.
Phil's blogpost on the subject: muircheartblog.wpcomstaging.c...
This video was formerly called What is Quantum Superposition?
/ computerphile
/ computer_phile
This video was filmed and edited by Sean Riley.
Computer Science at the University of Nottingham: bit.ly/nottscomputer
Computerphile is a sister project to Brady Haran's Numberphile. More at www.bradyharan.com
I think this deserves a continuation to explain how quantum computing really computes.
came here to say the same. we need round two! how do you go from the superpositioned waves to an actual computation?
@@ByteMe1980 I came here to say the same thing too!
He kinda said he didn't know in the video. He said he isn't part of it but there are people working on algorithms to try and use these superpositions as waves.
5km view just to see how it looks like. With the factorization as example (Shor algorithm)... You enter the 2 numbers as wave "periods", once collapsed it will give you the "period" of when the two combined numbers are the same, which is the factor.
Imagine 2 gears with different numbers of teeth, the common factor will be the turns you have to do so both gears are at the same position as the start.
A classical computer hast to count the turns until it repeats. A quantum computer does it "instantly".
Of course "enter" and "give" involves a lot of math and calculations, but basically all the computation is done by manipulating wave properties in a clever way.
@@ByteMe1980 The basic idea of (some) quantum computing is that you expand the state of the qubits into a particular superposition, apply some algorithm, and then carefully undo the initial expansion with some inversion operation so that you measure something with certainty (or near certainty in many cases). Now, as Phil says at the start, it can be mischaracterised if you're not careful, as there's nothing magic about this (the way you set up the quantum state is very precise), and very often the superposition you set up does not have some trivial one-to-one correspondence with the values of classical bits as is typically descibed, but I wouldn't say it's far off.
5:53 'I don't have infinite energy'
Your videos suggest otherwise, Phil
We're fighting the second law but we aren't winning.
There's only just so many After Eight mints in that box, Eline.
He has a way of explaining something in a way that makes it relatively easy and impossible to understand at the same time. I mean it as a compliment.
So you are basically in a super position of eureka and total confusion at the same time.
@@3dlabs99 Yes, exactly. Only thing left to figure out is how to get a nobel prize out of this situation.
I followed and lost it and still stayed with him.
I very much take this as a compliment! Thank you so much. Contrary to popular belief, confusion is not necessarily something to be avoided in teaching. See the blog post linked in the video information.
Philip (Moriarty, speaking in video)
Then maybe you don't realize what you said. If the student hasn't learned...
I love how scientists get so excited when they say “I don’t know”
"If you can answer that question, you get a Nobel Prize."
That's a pretty funny answer to the question "Why?" about a part of quantum physics that does confuse people.
@@ZT1ST Of course it confuses people. It confuses physicists too! The measurement problem is unresolved. Would you prefer that I made up some explanation or pretend that parallel universes are the answer (on the basis of zero empirical evidence)?
Philip (Moriarty, speaking in video)
"I don't know... yet" is like job security or printing money to scientists. Really plays into that part of our lizard brains that likes random chance, like what if we find out and it's really valuable.
Just to make it sound a little less noble, a little more real.
That's fair enough though. Scientists LIVE for the day they can find out what they don't know, its almost as great as when you actually figure it out :D
@@CaptainWumbo
I whish this was true...but the reality is not a lot of money is lining up to fund research where the answer is "I don't know" to the question "How will this be valuable"
What I appreciate most about prof Moriarty is that he's so vocal about science not knowing - and how that doesn't matter for the science. Why? We don't know. Period. We may know one day, but then there's likely to be more stuff underneath or within that we don't know - science rarely runs out of questions, but we learn more all the time.
Well said!
I find it similar to how you can still do useful work with a car even if you don’t know how it works. You can still drive it and carry stuff. If you take the time to learn how it works, you can manipulate it to be more efficient or effective, or to build new cars that are better suited for specific tasks.
Even if you don’t know why the wave collapses to one of those states, you can still exploit its properties to do useful work.
How has Holmes not captured Moriarty, right there in Nottingham?
Science is art of finding relevant questions.
"Quantum mechanics is simple: it's just wave interference and harmonics."
"So we can measure a quantum wave and all its harmonics?"
"No, you can't do that, it'll stop moving and lose all its harmonics."
"Why?"
"No one knows."
No we do know. Adding energy to the system, by shooting a photon at it or having it hit a detector collapses the harmonics into a single macro state. Like putting your finger on the bass string directly changes its timbre.
@@ac.creations That's not explaining what happens though, that is just an explanation of what can cause a collapse, but not what happens during the collapse.
There are two theories.
Copenhagen Interpretation: The waveform instantly collapses into one state upon observation.
Many-Worlds Interpretation: The waveform never collapses. Upon observing, the observer becomes "entangled" with the waveform, meaning that both the original waveform and the observer's waveform are in one big superposition. The waveform looks like it collapses to you because you are now part of it.
@@samuelthecamel Why does that lead to "many worlds"? Ok, so I'm entangled with a waveform, why does that make the universe split (I know that's not accurate terminology, maybe "branch" is better)?
Neither of these approaches explain why this happens, only what happens
That slinky demonstration is the best example I've seen to explain harmonics in the string! Thank you! I was not expecting timbre to be related to quantum computing
It would also be a really simple way of showing how the energy in a photon is proportional to the wavelength. Hopefully some middle school teacher steals it.
@@t_ylr that's an amazing connection to make.
Can light waves have a similar "timbre"? Can you somehow add light waves of different frequencies together?
@@ButchMarshall Absolutely. In fact, *all* real light waves have a spread of frequency. It might be very, very, very narrow but the only time we have a pure frequency (i.e. a single value that is defined right down to an arbitrary number of decimal places) is for a wave that lasts an infinite amount of time (or spreads across an infinite amount of space) without being disturbed. But this is a mathematical idealisation....
Philip (Moriarty, speaking in video)
@@thequantumworld6960 so what stops us from making qubits using lasers and measuring device? Why the need for these superconducting super chilled interfaces into the qubit?
@@ButchMarshall It's likely a lot easier to control electrons than photons. They're computing with the bits, you have to find ways to perform operations on them and I'm guessing it's a lot easier to do that with electrons. If anyone has any better ideas I'd like to know if there's a real answer.
Every time he says "Monke" I get the biggest smile on my face haha
Love ya Professor, keep it up!
This is hands down the best video I ever watched on Quantum Mechanics, and I’ve watched a fair few. I love how Phil isn’t afraid to use analogies, but also tells us where the limits of the analogy are, and his dedication to being accurate even in his layman’s explanations reminds me of Richard Feynman’s approach.
Precisely! It's both simple and intuitive while not shying away from the big picture. Vey Feynman-like Indeed. Takes a lot of skill to come up with explanations like that. I wish I had that kind of talent. I tend to complicate things more when I try though.
Agreed. It's the first time I have seen an explanation of superposition that actually made sense :D
Insanely good video, on of the best of prof Moriarty! I never heard QM being condensed into such a simple and intuitive explanation.
This is incredibly easier to understand than before. Thank you for being well versed.
Fantastic use of Bass to explain superposition. Davie504 would be proud of you.
He wrote a whole book on that :) Turn up to eleven!
Everyone needs to SLAP the like button on this video ;)
I'm glad I'm not the only one frustrated with all the "QC explained in 10 minutes" videos. They all can be summarised as "I'll restate the confusing part and pretend that's the explanation for it."
I've learned more about Quantum Computing and Quantum Mechanics from a few computer file videos with Professor Phil Moriarty than I have from a lifetime of 'documentaries' and hyperbolic articles. His ability to elucidate and cut through the bull is fantastic. Genuinely appreciate his enthusiastic but humble approach to communicating traditionally mystified topics.
I have always loved this guy's passion and overall demeanor when he talks about his field of science.
He gets me pumped up when he gets so excited and passionate about whatever he's talking about.
Man I wish this video was longer. Would love to hear Phil go into more detail about this. Very interesting topic plus Phil is just the best.
Nice to see someone aging right along with me.
I felt this in my bones.
Age is just another wave function unfortunately.
rip
"Computer scientist really like their matrices and vectors"
Oh, finally an explanation that ill unders- nope its music
...but it's all the same! Wave mechanics and matrix mechanics are the same principles, just expressed in a different "language". A function can be thought of as a vector in a high dimensional space...
Philip (Moriarty, speaking in video)
Finally someone who does not resort to saying "..some magic happens...and presto...".
I learned something today. Thank you mr. Moriarty.
As someone who is confused by many videos on quantum mechanics etc I found this helpful but I wish it had gone further. How can we derive a one or zero from a wave when making the measurement seem arbitrary? I don’t get it, he said over and over.
I think that was skipped because the answer can be quite involved and mathsy. That's the weird part he alludes to, ie. the truly quantum part, rather than just superposition/wave mechanics. There's no reasonable analogy you can make from a quantum superposition collapsing to a guitar string oscillating. A qubit's superposition collapsing into either zero or one is pure quantum mechanics: energy levels turn out to be discrete, not continuous, so in this case it must pick one energy level or the other (which we label as zero or one) when we measure it, not somewhere in between. How that measurement actually physically works in a quantum computer I'm not sure, but there are lots of ways to disturb a quantum superposition and it happens all the time without our involvement.
On the maths side of things, most of the time we can't derive a one or a zero from a qubit's superposition with certainty, even if we have perfect knowledge of its state. We can only derive the _probability_ that a qubit will be zero or one when we measure it. What state it actually ends up in is random, based on that probability distribution (or at least, that's how it appears to us). The clever and difficult part of quantum algorithms is manipulating the qubits/superpositions so that they interact in such a way that they calculate something useful while also ensuring when we do a measurement we get the answer we want with certainty _or_ with high enough probability that if we run the algorithm a few times we end up with the right answer. Honestly, I only have a vague high level understanding of this because the nitty-gritty of how it works involves a lot of linear algebra. For a concrete example, minute physics has a great video on Shor's algorithm, and 3blue1brown has some very helpful explanations of the maths involved.
(context: I took a couple quantum computing units I didn't have enough time to understand in detail, so I'm far from an expert)
The ones and zeroes are measued with probabilities that are determined by the quantum algorithm. In an ideal scenario, the probability of measuring the correct result is 100%, but it usually isn't quite that simple.
It's a 15 minute video. You're not going to get a detailed, blow by blow analysis of quantum computing/quantum mechanics in 15 mins! I teach the first semester of our Quantum World module, which is 12 weeks of tuition, involving a total of 37 videos, eight worksheets, 12 in-person teaching sessions, and many other tutorial/exam/revision sessions...
...and still we barely scratch the surface of quantum mechanics. One key issue with TH-cam "edutainment" is the idea that complex, complicated questions have straight-forward answers that can be expressed in a clickbait video title. As Feynman said when asked to describe "in a few sentences" the work for which he won the Nobel Prize: "If I could explain it in a few sentences, I wouldn't have won the Nobel Prize."
The blog post linked in the video information (and the other posts/articles in turn linked in there) provide a lot more background and information. But the intellectual heavy lifting when it comes to understanding a subject like QM has to come from the learner, not the teacher.
Sorry for such a long reply but this is an aspect of TH-cam edutainment that particularly exercises me!
Philip (Moriarty, speaking in video)
@@thequantumworld6960 Really interesting to hear that first hand Philip. You and some of the others over at Sixty Symbols did a video years ago about the state of high school physics and the public perception of the subject... it would be interesting to hear more from you about what you think this big world of TH-cam physics/maths/science videos needs to be careful about and how one should go about getting the balance between detail and engagement correct.
hands down my fav computerphile video of all time
Does Professor Moriarty have his own TH-cam channel? I'd be interested in seeing some longer form content from him. I always enjoy the videos with him in them.
I'm also curious
I think he does but can't find it.
Hi, @PBJ AND A HIGHFIVE.
Thanks so much for asking. Yes, I have a channel, "The Quantum World", which features videos for our 2nd year undergraduate physics course of the same name. Click on the avatar...
Philip (Moriarty, speaking in video)
@@thequantumworld6960 I'm glad you checked the comments and responded! Just subscribed.
The university could find a way to publish his lectures like MIT is currently doing in their channel.
I've seen most of the videos on TH-cam tying to explain what superposition is and this is by far the best explanation. Thank you so much!
This is the best explanation of quantum phenomena I have ever seen. Outstanding.
Always such a pleasure seeing Professor Moriarty in action. I'm sure he could be reading his weekly shopping list and make it sound exciting...
"This is one Hertz ... this is two Hertz ... this REALLY hurts"
Finally a legit explanation on the topic of quantum physics, and not some garbage pushed by journalists who don't have a clue. Thank you so much!
As a former jazz pianist with excellent ear but no background in science, this video makes perfect sense. If you play a low bass note on a piano, you can hear almost as much mid and treble as you can hear bass, and if you listen carefully you can hear all the harmonic notes in the sequence, but everything must all balance out to being most 'probably' (definitely) the note that it is.
Sean and Phil: bravo for agreeing to tackle this complicated topic and doing or so well.
I understood that from my experiments in audio DSP. This gives me the courage to think that if I delve deeper into this I may actually understand more. It's the first time I ever imagined that I might grasp what this is all about.
Moriarty is the king when it comes to intuitive explanations.
Very nicely done, and now you're playing bass, love it! Fantastic explanation along with your fantastic book! Thank you so much.
I had been struggling with understanding superposition until I watched this video. Thanks for making it very clear!
Loved this! I have never heard this kind of explanation of Quantum Computing before.
Incredible video. Incredible wealth of knowledge. You best believe I’ll be watching this multiple times
I'm emailing this video to my old quantum mechanics professor. Excellent explanation!!
What an excellent description. Well done.
Literal perfect timing, exactly the video i was looking for only uploaded hours ago :)
This might be one of my favorite Moriarty videos yet. I feel like I've said this about basically every single one that came out before this, though 😅
As a layperson who's been curious and studying the subject casually for some time, this really feels like a great sweet spot between the whole "quantum is wooonky" and "quantum is just plain math" attitudes you see here and there.
Absolutely fascinating.
Thank you for this amazing content
I enjoyed the video a lot! He is clearly enjoying the video-class and the topic! This are the kind of teacher that inspire students!
As a musician, I can know understand a representation of the concept of superposition. That was amazing Phil.
Thank you! This is the most interesting video on TH-cam. I keep rewinding again and again trying to understand this. I think I got understood some small bits 🙂❤️
This is just gold. Thank you so much for explaining that
Great insight Phil, please keep them coming !
This is an amazing explanation
This is so relatable. Best explanation of quantum computing ever!
This video reminded me why I love physics so much. Reality can be quite trippy.
That made that topic a lot clearer, Thank You
Love to see professor Moriarty, very down to earth
those were all amazing analogies that, except for one, i hadn't heard before, and that may be one of the best overall introductions to the subject i've heard. very fun.
Great segment.
This is the best video of quantum mechanics ever!
This is the best video on the internet
This video is a fantastic explanation of quantum systems!
Little did I know I'd find the best explanation of the Measurement problem on a Computer channel
Great explained!
i definitely understood it better than EVER before! is like polyphonic singing or overtone singing. treating particles as voice, they have many overtones but when meassured (meaning extracting energy from the particles to excite sensor and let it know there is something) the energy drops and falls into a single overtone. which will vibrate over time and make new multiple pletora of overtones again. soo cool!!!
That was probably the closest that I've ever been to nearly grasping the concept of quantum mechanics.
Professor Moriarty is so good at explaining this stuff.
Amazing explanation
Thank you Professor Moriarty!
Really well done. I learned much
I am a musician and using the harmonic series to explain this is perfect for me, thank you so much, I feel as if this video was almost made for me.
Amazing episode
The only ones who've ever attempted to describe how D-wave works are D-wave themselves, last time I checked. I've never been convinced by their explanations that what they're doing actually "works" and I've never seen an application for what their systems might be able to do for a business, certainly not versus a cost analysis.
Very well explained
also a great explanation of the measurement problem
Thank you for explaining that superposition does not mean "every possible position all at the same time." I have heard this description almost exclusively, and it never made sense to me. I chalked up the "doesn't make sense to me" as being inherent in the fact that it is quantum mechanics. While I, of course, don't understand quantum mechanics much better now than before... I do have a better handle on what questions I need to ask.
This video taught me a lot!
Love that we can see his music hobby on display; bass, amp, mike
Excellent video
I love this passion so much!
When I see Dr. Moriarty in the thumbnail I know I'm going to watch. Another excellent explanation, of course.
Is the monkey going to be featured in more videos? He was capable demonstration assistant.
2M congratulations......💯❤
Awesome explanation! Loved the example with the monkey. Thanks!
The course I took on acoustics gives me a little bit of a leg up on understanding this, as I knew precisely where Dr. Moriarty went with the accumulated sine waves expressing the timbre of the sound and how it applies to quantum computing. Imagine using the Fast Fourier Transform algorithm to assist in quantum computing. Very interesting video... hats off!
Great point! Fourier transforms are absolutely core to quantum mechanics (and therefore quantum computing.) See the blog post linked in the video information (and the links therein) for much, much more on Fourier's role in QM.
Philip (Moriarty, speaking in video)
Thank you so much!!! This in my opinion the one and definitive explanation about quantum mechanics which makes sense. It should be always explained this way in schools and in scientific publications for media. I’m a failed phisics student in 80’s and the main reason I quitted studying was the unbearable lack of connection between the models and how things work in the real world. Or at least it was how professors made it feel to me. Maybe I should have chosen engineering? 🤣
Those PC speakers should be in an archaeological museum. 10:10
I know! I love 'em!
Philip (Moriarty, speaking in video and owner of said speakers)
Thank you!! That was sooo helpful.
👏 great, please post more like this!
AMAZING! Thank you!
Maybe the physics we call "classical" are just narrowly applicable on specific energy levels, which are the levels we exist at -- whereas most of the universe is either near absolute zero, or at extremely high energy levels. And when we "measure" something, we're just reducing the temporal dimension (an "instant" to us) to where it behaves like the everyday physics we are familiar with. Sort of like how an ocean would appear like a solid if you interacted with it at 0.01 frames per second -- but behaves like a wavy liquid at 100 frames per second.... if that makes sense
That's a great analogy
yeah that's very similar to my own pet theory of what's going on. If we consider time as we understand it to be the 4th dimension of our existence with things like how gravity can bend "spacetime" and whatnot, electrons at a quantum level could be thought of as existing at a dimensionality beyond our 4 intuitive dimensions. If we also add to this the concept that time exists at the finest level with a discrete smallest unit of time (there exists a unit of time that cannot be divided), the issue of "probability" in measurement could be that when we attempt to measure the electron in our limited 4 dimensions we are actually "collapsing" 5 or more dimensions into just 4 to get our measurement, thus why the wave collapses into a single state since our action of measuring in 4 dimensions causes us to "hold constant" any other dimensions at play, thus destroying the harmonic (take a 2 dimensional wave-form, and force it into 1 dimension to take a measurement, you can't return now to the original 2 dimensional state) -- Wish I knew enough to work out this theory a little further, it's very interesting to think about.
I like that. Problems like this (I believe) probably depends heavily on relatively and our 3 dimensional observations. The energy levels we're used to interacting with also coexist with the only life (that we know of) in our small sliver of the universe..... so how do we know gravity doesn't bias our answers? The universe as we know it is mostly empty space, and we happen to be conducting all of our experiments really close to a fairly large mass, which as we know has an effect on our perception of time... Think about this, if a small object (baseball) is released next to another object with significantly greater mass (planet), the ball will appear to be getting closer to the planet (relative to the planet) and the planet will appear to be getting closer to the ball (relative to the ball), yet an outside observer will clearly see the ball moving towards the planet (the right answer?) The fact that there's 3 different conclusive answers to this one measurement isn't weird, it's relativity, but what happens if you can't become the third observer? How do we even know when we are the third observer when measuring something? Can you prove when we are?
I think I finally ... well, not get it, but, I'm pretty sure this is the closest to understanding how I should be thinking about the concept so far. Excellent video. Again professor Moriarty is inspiring me to use my uneducated software developer brain to really get my head around computer science, maths and physics. Thank you
Please make more of these videos.
What you neglected to explain is what exactly is special about quantum computing.
If we could do those calculations on any wave system - there are much easier ways to do it from pendulum systems to vibrating strings. But we don't see guitar calculators - so there is something special about quantum that allows certain types of parallel algorythms to work incredibly fast.
Getting definite flashbacks to Phil's book, When the Uncertainty Principle Goes to 11. He goes over a lot of this in the book.
That simulation is amazing
At 7:36, if Quantum Computers compute via wave interference, why aren't they just considered classical, analog computers? Wave interference is a very classical physics phenomenon being used for computing. You say the word "states", but does that just mean the analog, waveform shape at a particular time?
At 8:57, is a "Quantum" explanation, but it seems that the classical, analog waveform being measure is a resonance *"state"* (or one of the overtones). Again, this still seems very classical. In terms of "Quantumness," is it because we just don't know which resonance *"state"* will be measured?
Sincerely, I think this is the thing missing in this video. - a clearer explanation of how quantum computing computes.
For what I understood the quantum computation comes nt from "waveform shape at a particular time" but the multiple combinations of the various single"waveform shapes at a particular time". That is why he reject the notion that quantum is the result of infinite computation.
The best I could grasp is that quantum computing aggregates the probabilist result of the many variation of the waves a particle - qubit - can generate from a given input.
Again, I mgiht be wrong, but whan Ive saw explained differently, the difference from clasical computing is that the output is either 0-1. In quantum the result comes in terms of probabilites: from all the positions that the wave represeting this qubit uccupied uring the calculation, which is the most probable output.
The benefit apparently comes from the fact that you can do with one qubit the calculations one could do with many bits.
@@mustavogaia2655 **
This would be like an analog computer that used overtone/harmonics of a fundamental wave to do computing.
**
But, if you get these random energy peaks or random position peaks when you measure the wave, can you build a computer based off of random number generation? i.e. qubits seem to just produce a random number when measured.
@@itsbs
Yes, but this would be considered a single quantum operation.
Again, not an expert, but the "peaks" are dependent on the function/matrix the is passed thru.
@@mustavogaia2655 **
I thought he just said "when measured" you get some different (maybe random) waveform.
I did a couple of weeks of quantum computing bare essentials in one of my classes for my CS degree 10+ years ago. Most of what I remember are the two superposed vector-states of the qubits rotating around a lot on the complex plane. All the rest of it faded into the ether.
I really wish there was a way to make whatever he was showing accessible to myself, since I am not able to actually see the visual and this stuff is extremely fascinating to me
Love the bass in the back.
Perhaps hands down the best description of quantum mechanics, suddenly it makes sense!
This guy is a GREAT 👍 DEMOSTRATOR. well done.
Really appreciated!
Not mentioned in video, but key to quantum computing is the ability to manipulate the probability of the collapsed wave function using microwave pulses. That's what quantum gates and circuits are for.
Yes, and hopefully we'll get to this in a future video. The entirety of quantum computing/quantum mechanics is not going to be explained in a 15 minute video...
Philip (Moriarty, speaking in video)
This is something I still wish to understand. From the start of learning about QC, I wondered what substituted the logic gates of classical computers with a device that can have countless states as output. Seems Im slowly getting there thanks to these and other videos.
I also have to think (I could easily be mistaken), we are currently coupling QC with classical ones, to make sense of the massive amount of information they generate?
Its easier (for me) to think of QC as standalone data output devices rather than a full blown computer unto itself? In a world were we have grown up around classical computers up till now. Seems nearly impossible to build a purely QC ground up with a user interface? I kinda think of it like a D/A A/D converter. QC into classical??? Then the obvious issue of needed bandwidth to pull this off. Dunno... Still trying to understand it all. Slowly but surely.
Love this guy.
I never realised that, as a slightly nerdy musician, all I needed to understand quantum theory was a slightly nerdy musician to explain it. Thanks fo that! 😁
This is a great video on quantum computing, so much of it is filled with the same hogwash we see in quantum mechanics of quotes of consciousnesses collapsing waves etc.
An expertly concise way of explaining how it is possible to utilize the wave. Always love some Moriarty.
Now I want a mint! "It's wafer-thin!"... love Phil, we don't get enough Moriarty.